1. Field of the Invention
This invention pertains to the alkoxylation of polyhydric initiators to prepare polyols useful in manufacturing rigid polyurethane foams. More particularly, the invention pertains to the use of catalytic quantities of urea for the alkoxylation of polyhydric initiators, in particular sucrose, and the rigid urethane foams made therefrom.
2. Description of the Related Art
It is known that crosslinking polyols which possess a high functionality, that is, a high number of free hydroxyl groups, can be used to produce polyurethane cellular products with good strength and dimensional stability. In general, the higher the functionality of the polyol used, the greater the rigidity of the polyurethane product. Thus, sucrose polyols having a functionality of 8 are of importance in rigid polyurethane foam preparation as disclosed, for example, in U.S. Pat. No. 3,763,111.
It is known to prepare sucrose polyols by reacting sucrose with alkylene oxide in an aqueous solution in the presence of sodium hydroxide. See, for example, LeMaistre, et al., J. Org. Chem., 13, p 782, (1948). U.S. Pat. Nos. 3,085,085 and 3,153,002 disclose a process based on this reaction in which sucrose is reacted at elevated temperatures with ethylene oxide or propylene oxide in a concentrated aqueous solution in the presence of potassium hydroxide catalyst. Such processes, however, are subject to undesirable secondary reactions such as partial hydrolysis of the alkylene oxide by the water used as the reaction medium, resulting in a product which is very dark in color. In order to provide a method which prevents discoloring and the other problems associated with those processes, U.S. Pat. No. 4,380,502 to Muller, et al. discloses the use of polyether polyols made by alkoxylating a mixture which is from 20 to 80 wt.% sucrose and 80 to 20 wt.% formitol. Muller, et al. teach that the polyether polyols prepared according to their process exhibit a color ranging from clear to yellowish.
U.S. Pat. No. 4,230,824 to Nodelman discloses a method for preparing a sucrose based polyether polyol which involves the use of a polyalkylene polyamine as both co-initiator and catalyst for the alkoxylation reaction of sucrose. Nodelman reports, however, that the resulting polyols are very high in color; i.e., up to 12 Gardner. In a later patent, however, U.S. Pat. No. 4,332,936, Nodelman reports a method for making polyether polyols from sucrose comprising dissolving the sucrose in a solvent such as dimethylformamide in the presence of a co-initiator and amine catalyst prior to alkoxylation. Nodelman reports that this process enables the reaction to be conducted at temperatures as low as 80.degree. C., producing a product with consistently low color; for example, a Gardner color in the 2 to 3 range.
Applicants have surprisingly found urea is an effective catalyst and co-initiator for the alkoxylation of aqueous sucrose solutions and results in polyol products which are much lower in color than prior art polyols; i.e., on the order of 1 Gardner.
Urea has been added to polyols for other purposes. U.S. Pat. No. 4,485,032 teaches that the addition of a minor amount of urea and/or substituted ureas to incompatible polyol blends often renders these mixtures compatible and produces a single phase liquid. Urea has also been added to polyols to prepare fire retardant polyurethane foams, as demonstrated in U.S. Pat. No. 4,481,308.
In order to avoid the mechanical difficulties encountered in mixing urea with polyurethane foams to produce a self-extinguishing foam, U.S. Pat. No. 4,459,397 to Richardson, et al. teaches that a polyether polyol exhibiting improved fire resisting qualities may be prepared by reacting a reducing sugar with propylene oxide to produce a polyether polyol which is then reacted with unsubstituted urea to chemically bind the urea thereto to produce a modified polyether polyol. Richardson, et al. also report, however, that attempts to react urea and a sucrose polyether polyol were unsuccessful.
DD 212,967 discloses polyether polyols useful in rigid polyurethane foams that are prepared by polymerizing a portion of the required amount of epoxide on urea or its derivatives and then adding polyoxypropylated glycerol-urea and KOH catalyst.
Tousignant, et al., in "Reaction of Ethylene Oxide with Urea," J. Org. Chem., 22, pp 166-167, disclose that the reaction of urea with alkylene oxides produces alkoxylated ureas and amino carbamates.